Split Program.cs DI registrations into per-namespace DependencyRegistration classes

Each root namespace (Audit, Common, Data, Environments, Features/Usage,
Identities, Organizations, Projects, Segments, Users, Webhooks) gets its
own static DependencyRegistration class exposing an Add<Name>Services
extension method, so Program.cs just wires them together instead of
listing every registration inline.

src/api/MicCheck.Api/Users/DependencyRegistration.cs

@@ -0,0 +1,14 @@
+using Microsoft.AspNetCore.Identity;
+
+namespace MicCheck.Api.Users;
+
+public static class DependencyRegistration
+{
+    public static IServiceCollection AddUsersServices(this IServiceCollection services)
+    {
+        services.AddScoped<UserService>();
+        services.AddScoped<IPasswordHasher<User>, PasswordHasher<User>>();
+
+        return services;
+    }
+}